1. Field of the Invention
The present invention relates to a structure having holes, and a method for producing the same. In particular, a nano structure made by using a method for anodic oxidizing Al according to the present invention is useful for a wide variety of applications including functional materials and structural materials for electron devices, memory media and memory elements. Specifically, the nano structure is effective for use in vertical magnetic recording media, patterned media, solid magnetic memories, magnetic sensors, and photonic devices.
2. Description of the Related Art
Some thin films, fine lines, and dots of metals and semiconductors that are smaller size than a characteristic length may show specific electric, optical and chemical properties by inhibiting movements of electrons. From such a viewpoint, interests on a material having a microstructure of below 100 nano meter (nm), i.e., nano structure, have been increased.
A method for producing the nano structure include semiconductor processing technologies including a fine pattern drawing technology such as photolithography, electron beam lithography, X-ray lithography and the like.
Other than such production method, there is one approach to provide a novel nano structure based on a naturally formed regular structure, i.e., a self-organized structure. This approach can produce a finer special construction than the conventional one depending on a fineness of the structure used as a base, and therefore many studies have been made.
Examples of the specific self-organized structure include an anodically oxidized alumina film (for example, see C. Furneaux, W. R. Rigby & A. P. Davidson “NATURE” vol. 337, P147 (1989)). An Al plate is anodically oxidized in an acid electrolyte to form a porous oxidized film. This porous oxidized film is characterized by a special geometric structure where highly fine columnar nano holes (microholes) 14 each having a diameter (2r) of several nm to several hundreds nm are arranged in parallel at a space (2R) of several tens of nm to several hundreds nm as shown in FIG. 2A. The columnar nano hole 14 has a high aspect ratio, and has excellent uniformity in a diameter profile. The diameter 2r and the space 2R of the nano hole 14 can be controlled to some degree by adjusting a current and a voltage upon the anodic oxidation. An anodically oxidized alumina film is produced on an Al plate 21 via a barrier layer 22.
A wide variety of applications have been tried by focusing on the special geometric structure of the anodically oxidized alumina nano hole. The details are described by Masuda. For example, the anodically oxidized film is used as a coat utilizing its abrasion resistance and good insulation, or used as a filter by peeling the coat. Furthermore, various applications includes coloring, a magnetic recording medium, an EL light emitting element, an electrochromic element, an optical element, a solar battery and a gas sensor using a technique of filling a metal or a semiconductor into the nano hole, and a technique of replicating the nano hole. In addition, a wide range of applications including quantum effect device such as quantum fine line and MIM element, and a molecular sensor using the nano hole as a chemical reaction site are expected (Masuda, Solid State Physics, 31, 493 (1996)).
The above-mentioned method for producing the nano structure by the semiconductor processing technologies has problems of a poor yield and expensive apparatus. There is a need to provide a simple method for producing the nano structure with good repeatability.
In view of the above, the self-organization method, especially the Al anodic oxidation method, has an advantage that the nano structure can be produced easily and controlled well. Typically, these methods can provide a large area nano structure. However, when an aluminum layer is formed on a substrate, and is anodically oxidized, tightness between hole walls and the substrate may be poor.
FIGS. 2 and 3 show a conceptual sectional views of conventional alumina nano holes on an Al plate (film). FIG. 2A is a sectional view showing an Al plate oxidized on an anode. FIG. 2B is a sectional view showing the Al film on the substrate not completely oxidized on an anode. FIG. 3A is a sectional view showing that the anodic oxidation is terminated with a barrier layer remained. FIG. 3B is a sectional view showing that the barrier layer is removed by dry etching and the like.
The conventional anodically oxidized alumina nano holes are provided only on a surface of the Al plate (film) as shown in FIGS. 2A and 2B, and their applications and forms are thus limited. For example, Al has a melting point of 660° C., and the nano holes formed on the Al cannot be heated at 660° C. or more. In order to use the nano holes as the functional material in various aspects, it is desired to provide a technology for forming the anodically oxidized alumina nano holes on a substrate having a high melting point.
In order to apply the anodically oxidized alumina nano holes to an electronic device and the like, it is desired to provide a technology to embed an enclosing material and to form the enclosing material connectable to the under layer. If the anodically oxidized alumina nano holes can be formed uniformly and stably on the under layer including a good conductive material such as metals, it is possible to form the enclosing material in the anodically oxidized alumina nano holes by controlled electrodeposition, whereby the application can be expected to be broaden.
As an example of forming the anodically oxidized alumina nano holes on the substrate, Japanese Patent Laid-Open No. 7-272651 discloses a technology for “forming an Al film on a Si substrate, altering the Al film to an anodically oxidized film, removing a barrier layer at a bottom of a nano hole part, forming a metal (Au, Pt, Pd, Ni, Ag, Cu) layer capable of forming an eutectic alloy with Si of the Si substrate exposed at the bottom of the nano hole to grow Si needle crystal by a VLS method.”
In the technology, the barrier layer at the bottom of the nano hole is removed after the Al film is anodically oxidized, in order to penetrate the nano holes to the Si substrate. As the method for removing the barrier layer, there are cited a method for etching with chromic acid-based etching liquid, and a method for connecting the Si substrate and a counter electrode with an external wire after the anodic oxidation is completed, and for holding the structure in the liquid.
Through intense studies by the present inventors, it is found that after the Al film is oxidized on the anode across a total film thickness and the barrier layer remains as shown in FIG. 3A, it is very difficult to complete the anodic oxidation with good repeatability.
Especially when the under layer is disposed under the Al film, and the substrate as the under layer or the under layer are made of a low reactive material, and the anodic oxidation proceeds in the state shown in FIG. 3A, the barrier layer is deteriorated or lost in a very short time and the electrolyte is contacted with the substrate (or the under layer) resulting in an electrolyte decomposition. Even if the anodic oxidation is terminated immediately before the state shown in FIG. 3A, depths of respective nano holes may be deviated to some degree. Accordingly, it is difficult to produce the structure having a uniform barrier layer remained over a wide range as shown in FIG. 3A.
The structure with the barrier layer remained as shown in FIG. 3A may realize in a part of the substrate. In this case, when the barrier layer is then removed, the diameters of the nano holes near the removed part lack linearity and become discontinuous as shown in FIG. 3B, and the shapes of the nano holes are largely different.
In particular, if the nano holes are deep, thicknesses and a proceeding degree of the anodic oxidation become easily deviated. It is difficult to give the barrier layer with the uniform thickness, and it is almost impossible to remove the barrier layer by the dry etching and the like.
There is no description about the production of the anodically oxidized alumina nano holes using noble metal and carbon as the under layer. It is contemplated that if the under layer is made of these low reactive materials, water is started to be electrolyzed once the under layer is anodically oxidized and foams are produced, which breaks the anodically oxidized film.
One object of the present invention is to provide a structure having holes penetrating to the predetermined depth area. Other object of the present invention is to provide a nano structure having nano holes with excellent linearity and diameter uniformity where bottoms of the nano holes are penetrated to an under conductive metal layer, and a method for producing a nano structure to form the anodically oxidized alumina nano holes uniformly and stably.
Still other object of the present invention is to provide a nano structure and a production method therefor providing excellent tightness between an alumina nano hole layer and the substrate, or between the alumina nano hole layer and the under metal layer when the alumina nano hole layer is provided on the substrate via the under metal layer. The nano structure of the present invention has excellent tightness and therefore is preferable especially when a polishing step is conducted after the nano hole production, or when a stress is applied upon the use of the structure.